1. Field
The present invention relates to data communication, and more particularly to techniques for acquiring and tracking signals (e.g., pilots) in a CDMA communication system.
2. Background
Wireless communication systems are widely deployed to provide various types of communication such as voice, data, and so on, for a number of users. These systems may be based on code division multiple access (CDMA), time division multiple access (TDMA), frequency division multiple access (FDMA), or some other multiple access techniques. CDMA systems may provide certain advantages over other types of system such as increased system capacity. A CDMA system is typically designed to conform to one or more standards, such as IS-95, cdma2000, IS-856, W-CDMA, and TS-CDMA standards, all of which are known in the art and incorporated herein by reference.
A terminal in a CDMA system needs to acquire signals transmitted from one or more base stations in order to receive or exchange data. In many CDMA systems, a pilot is transmitted from each base station to assist the terminals perform a number of functions. The pilot is typically generated based on a known data pattern (e.g., a sequence of all zeros) and using a known signal processing scheme (e.g., covered with a Walsh code of zero and spread with a known pseudo-random noise (PN) sequence). For IS-95 and cdma2000 systems, each base station is assigned a specific PN offset that is different from those of neighboring base stations to allow the terminals to distinguish the individual base stations. The pilots from the base stations may be used at the terminals for acquisition, frequency and timing synchronization, channel estimation, coherent demodulation of data transmission, and so on.
The acquisition of a pilot is made challenging due to various phenomena. First, the pilot is distorted by noise and interference in the communication link, which can make detection of the pilot difficult at a terminal. Second, due to obstructions and artifacts in the transmission environment, a transmitted pilot typically reaches the terminal via a number of signal paths, and the received signal at the terminal normally includes multiple instances of the pilot at various amplitudes, phases, and delays. Third, since the terminal may not be certain of its frequency relative to the system, the received signal may be downconverted with a frequency error that needs to be acquired and tracked. And fourth, movement by the terminal creates a Doppler shift in the frequency of the received pilot, causing the pilot's frequency to be offset from a nominal (expected) value. The signal instances in the received signal may thus be associated with different (and possibly large) frequency errors due to downconversion frequency error and/or Doppler frequency error.
There is therefore a need in the art for techniques to effectively acquire and track signals (e.g., pilots) over frequency errors in a CDMA communication system.